1. Technical Field
The present invention relates to an electret, and more particularly to a polymeric electret film and method of manufacturing the same. The polymeric electret film obtained by such method is capable of remarkably improving the polarized initial surface potential and greatly lower the surface potential decay rate.
2. Description of Related Art
An electret is broadly defined as a dielectric material, which exhibits an external electric field in the absence of an applied field. The term “electret” is used as a generic name for the materials which can retain static electric charges for the long-term period. Electret materials can be easily found in our daily life. Today, most electrets are made from dielectric materials, e.g. polypropylene (PP), fluoropolymers, fluorinated ethylene-propylene (FEP), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), etc. These dielectric materials can permanently retain the static electric charges after they are electrized, thus existing as the so-called electrets.
Electrets can permanently retain two types of space charges, one being dipole charges and the other being real charges, both of which exist simultaneously in the electret. Real charge electrets are those that possess an injected or imbedded charge within the dielectric. Dipolar electrets, as their name suggests, are formed by the orientation of dipoles (i.e. polar groups) within the dielectric. Real charge electrets are typically formed by techniques that deposit or inject charge directly into the dielectric material. Dipolar electrets are formed, or polarized, by the application of an electric field to the material either at ambient temperature, or by heating the material to a higher temperature while applying an external electric field and then cooling to some lower temperature while the external field is maintained. In dipolar electrets, the reorientation of electrical polarization can only be achieved at temperatures where the dipoles are mobile. For most polymers of interest this occurs above the glass transition temperature. Dipolar electrets can also be formed by charge injection techniques wherein the electric field due to the imbedded charge causes dipole reorientation.
Real charges include surface charges and space charges. Surface charges deposits at or near the surface of dielectric materials. As being exposed to the ambient environment, surface charges are difficult to retain and usually temporarily stay on the dielectric materials. On the other hand, since space charges are retained inside the dielectric materials and are unlikely to lose as compared with surface charges, they can be retained in the dielectric materials permanently.
There are two types of real charge electrets, one being homo charge electrets and the other being hetero charge electrets. Hetero charge means that the polarity of the space charge is opposite to that of neighboring electrode, and homo charge is the reverse situation. Under high voltage application, a hetero charge near the electrode is expected to reduce the breakdown voltage, whereas a homo charge will increase it. After polarity reversal under ac conditions, the homo charge is converted to hetero space charge.
While there have been many approaches to charging electret materials, three major techniques are discussed herein:
1. Corona charging, as its name implies, involves the application of a corona charging to the implantation of charge in a dielectric material. Corona charging relies on the breakdown characteristics of the gas present in the gap between a pair of electrodes. The majority of the energy dissipated within the corona charging goes to the excitation of the gas. These charge carriers deposit charge on the dielectric surface at depths of only a few nanometers. Over time, charge trapped at the surface can move into the bulk and become retrapped at depths of several microns.
2. Thermal charging, as its name implies, involves the application of an electric field to a dielectric material at elevated temperature and subsequent cooling while the field is maintained. In electrets prepared from polar dielectrics, where the electrodes are vapor deposited directly on the dielectric
material, the thermal charging gives rise to dipole orientation. However, the use of external electrodes results in air gaps at the dielectric/electrode interface that can lead to very complicated charging phenomena. Electrets made using this charging method are called thermoelectrets.
3. Electron beams charging, as its name implies, have been used for the charging of film electrets, but typically not used for charging nonwoven or fibrous filtration media. Charge implantation with low energy electron or ion beams relies on the generation of a secondary electron cascade as a result of scattering of the primary beam within the bulk of the dielectric. Low energy secondary electrons and the slowed down primaries become trapped within the dielectric yielding an electret state, which depending on the material can have a very high stability. High-energy electron and ion beams (i.e. ionizing radiation) do not work well for electret charging because of the chemical damage caused to most dielectric materials as a result of radiation exposure. The damage leads to induced conductivity that destabilizes the implanted charge leading to recombination of positive and negative centers.
Electrets are extensively applicable throughout various industries including exercising equipment, acoustics, optics, medical treatment and electrics. They are photoelectrically used for touch screens and X-Y positioning applications. Their medical applications include audiphones and filter masks. For electroacoustic use, electrets can be seen in super slim loud speakers (SSLSs), cap speakers, amplifiers, microphones, earphones, and voice transmitters. In addition, electrets are also widely used in piezoelectric power generators, switches, motors, power generators, various transducers, high-voltage power sources, detectors and solar batteries.
Recently, electrets attract great attention as a biomedical material that contributes the so-called electret effect. For instance, since the human vessel wall is negatively charged, the negative charge deposition of electrets may be used to improve blood compatibility of polymers, thereby providing antithrombotic effect and facilitating growth of bones and synthetic membrane texture. Another important breakthrough of electrets is their application to electrophotography, which contributes to the development of electrostatic recording technology. Meantime, the electret effect has been found in important biopolymers, such as protein, polysaccharide and some coenocytes. In addition, many important biomolecules, such as haemoglobin and deoxyribonucleic acid (DNA), may have various polarized and charge storage areas.
A charged electret is in fact a polarized dielectric of a metastable state with a relatively long relaxation time. However, when the additional electric field is removed, the charge storage volume is gradually reduced and the charges decay along the exponential curve gradually. Under the room temperature, the type of electrets dominates how its polarization remains while a relatively high temperature can lead to quick decay of electret's charge storage volume. Hence, it would be an important issue to improve decay of electret's charge storage volume in high-temperature environment.
Electret polymer materials are required to be long-term stable and less sensitive to moisture or chemicals. Traditionally, while hydrocarbon polymer materials, such as polypropylene, polyethylene or polycarbonate, are relatively inexpensive and processible, and have good chemical resistance as well as mechanical properties, as electrets, they suffer from serious decay of charge storage volume and shortened service life, thus being incompetent for long-term effective applications. Perfluoropolymers, such as fluoropolymers, fluorinated ethylene-propylene (FEP) and polytetrafluoroethylene (PTFE), do have long-term stability, but are expensive and insoluble to solvents, thus being less processible and having their application scope limited. Therefore, there is a need for a material, when used as an electret, has long-term stability and is less sensitive to moisture or chemicals, wherein the electret shall have significantly improved decay of charge storage volume in high-temperature environment.
U.S. Pat. No. 4,046,704 discloses an electrets film made of poly-3,3-bis(chloromethyl)-oxacyclobutane with a thickness of 200 μm with initial surface potential approximately 600V when disposed in an electric field of 2000V at 160° C. and then cooled to room temperature. The surface potential decay of the film 30 days from polarization is not obvious but convincing data are again not provided. The initial surface potential of the film is also unclear.
In addition, U.S. Pat. No. 5,384,337 discloses a binder mixture having PTFT as electret particles, PU, and DMF. A matrix of fibers is impregnated with the mixture and cured, whereby the electrets are substantially uniformly distributed throughout the matrix to produce an electrostatic porous material. However, U.S. Pat. No. 5,384,337 does not disclose any data regarding surface potential decay, resulting in the performance of the electrostatic porous material remaining unknown.
Therefore, it is desirable to provide a polymeric electret film capable of remarkably improving the polarized initial surface potential and greatly lower the surface potential decay rate.